1. Technical Field
This invention relates to the technology of detecting degradation of catalytic converters used to modify the emissions of an automotive internal combustion engine, and more particularly to systems, using such technology, contained on-board an automotive vehicle.
2. Discussion of the Prior Art
A monitoring system, often referred to as OBD II, has been mandated by the State of California to cover catalytic converters on all vehicles starting in 1994. This law requires that the monitoring system detect when the catalytic converter has dropped essentially below 50% in conversion efficiency for hydrocarbons in the exhaust gas.
Prior art approaches to meeting such mandate have focused on the use of oxygen sensors [as discussed in copending U.S. patent application Ser. No. 07/741,881, assigned to the assignee of this invention] to provide a signal that corresponds with the oxygen storage capability of the catalytic converter. In such prior art, it is presumed that there is a correlation between catalyst oxygen storage, sensed signal features, and catalyst performance; often there is no such correlation.
The author herein is not aware of any prior application of hydrocarbon sensors to an on-board monitoring system of an automotive vehicle for monitoring the efficiency of its catalytic converter and thereby meet such mandate. However, hydrocarbon sensors have been used heretofore in large scale testing, such as for smoke stacks, to give an indication of the hydrocarbon content.
In hydrocarbon sensors known to the prior art, either conductivity or calorimetry can form the basis for providing a signal. In a semiconductor hydrocarbon sensor device, a material absorbs combustible gases (hydrocarbons and/or CO), which absorption changes the material's conductivity and thus provides an indication of the hydrocarbon in the gas to which it is exposed. In a calorimetric device, thermocouples are deployed on a divided sensor substrate, one portion of which is catalytic. Combustion of the surrounding gas by the catalytic coating will raise the temperature of such portion greater than the temperature of the other portion; comparison of the temperatures of such thermocouples gives a linear indication of the hydrocarbon surrounding such sensor.
It is difficult to apply such prior art hydrocarbon devices to on-board detection requirements for an automotive vehicle because: (i) they possess no reference to evaluate the sensor signal; (ii) the devices are bulky and generally inaccurate; and (iii) they cannot operate at the higher temperatures of automotive exhaust gases or operate continuously.